1. Field of the Invention
The present invention relates to surgical procedures that alleviate central retinal vein occlusion (CRVO), and more specifically, it relates to a multifunctional cannula for use in such CRVO procedures.
2. Description of Related Art
The central retinal vein (retinal vein) is a short vein that runs through the optic nerve and drains blood from the capillaries of the retina into the larger veins outside the eye. The anatomy of the veins of the orbit of the eye varies between individuals, and in some the central retinal vein drains into the superior ophthalmic vein, and in some it drains directly into the cavernous sinus. The central retinal vein is the venous equivalent of the central retinal artery, and like that blood vessel, can suffer from occlusion (central retinal vein occlusion), similar to that seen in ocular ischemic syndrome. Since the central retinal artery and vein are the sole source of blood supply and drainage for the retina, such occlusion can lead to severe damage to the retina and blindness.
Each year, about 60,000 people in the United States experience a blockage of the major exit vein of one of their eyes. Worldwide, this number is, undoubtedly, many times larger. The blockage decreases blood flow to the retina which results in a significant reduction of oxygen to the visual cells in the retina, causing vision impairment. The visual field becomes blurry or worse and is reduced to a yellow formless disc. If blood flow is severely impaired, blindness can ensue. This condition is formally known as “Central Retinal Vein Occlusion” (CRVO).
A refinement of a surgical procedure that has been recently created by an ophthalmologist is desired to alleviate this condition. The procedure currently requires three incisions into the eye and the injection through one of the incisions of a blood clot dissolver, known as tissue Plasminogen Activator (tPA), into one of the retinal branch veins. The purpose each of the three incisions are to (i) illuminate the retina, (ii) insert a syringe needle (more properly, a cannula) for delivery of the tPA solution and (iii) remove the vitreous humor and its replace it with a saline solution.
A number of issues arise when utilizing tPA to remove a central retinal vein occlusion. It is unclear whether the tPA works by dissolving the clot chemically, or if the overpressure caused by the injection dislodges the jet mechanically. If the latter is the case, then the clot may reattach itself elsewhere in the body. If the pressure in the retinal system of veins could be measured during the tPA injection, then a sudden drop in pressure suddenly during injection would indicate that the clot was probably dislodged. The problem is how to measure the pressure in the retinal system of veins. If the pressure is measured in the syringe pump outside the eye, changes downstream of the cannula may not be observable, given the high impedance to flow presented by its narrow tip. If pressure downstream of the cannula cannot be monitored while the tPA solution is being injected, an apparatus that can stop the injection for a reasonable length of time during the procedure (about 30 seconds) for pressure equalization to take place between the vein and the pump is desired so that an accurate measurement can be performed.
Although the prior art procedure is often effective, it is desirable to reduce the number of incisions and to overcome the apparent issues.